Update Dpi and scale factor computation

Remove pixelScale() in favor of logicalBaseDpi(). Compute scale factor
based on logical DPI and logical base DPI, or optionally based on the
physical DPI.

Add policies for running the scale factor and adjusting the logical
DPI reported to the application. The policies are set via environment
variables:

  QT_SCALE_FACTOR_ROUNDING_POLICY=Round|Ceil|Floor|RoundPreferFloor|PassThrough
  QT_DPI_ADJUSTMENT_POLICY=AdjustDpi|DontAdjustDpi|AdjustUpOnly
  QT_USE_PHYSICAL_DPI=0|1

Done-with: Friedemann Kleint <Friedemann.Kleint@qt.io>
Task-number: QTBUG-53022
Change-Id: I4846f223186df665eb0a9c827eaef0a96d1f458f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 210 insertions, 24 deletions

diff --git a/src/gui/kernel/qhighdpiscaling.cpp b/src/gui/kernel/qhighdpiscaling.cpp
index 95790b96a9..7ffbfbe1e5 100644
--- a/src/gui/kernel/qhighdpiscaling.cpp
+++ b/src/gui/kernel/qhighdpiscaling.cpp
@@ -46,6 +46,9 @@
 #include <private/qguiapplication_p.h>
 
 #include <QtCore/qdebug.h>
+#include <QtCore/qmetaobject.h>
+
+#include <algorithm>
 
 QT_BEGIN_NAMESPACE
 
@@ -56,6 +59,18 @@ static const char legacyDevicePixelEnvVar[] = "QT_DEVICE_PIXEL_RATIO";
 static const char scaleFactorEnvVar[] = "QT_SCALE_FACTOR";
 static const char autoScreenEnvVar[] = "QT_AUTO_SCREEN_SCALE_FACTOR";
 static const char screenFactorsEnvVar[] = "QT_SCREEN_SCALE_FACTORS";
+static const char scaleFactorRoundingPolicyEnvVar[] = "QT_SCALE_FACTOR_ROUNDING_POLICY";
+static const char dpiAdjustmentPolicyEnvVar[] = "QT_DPI_ADJUSTMENT_POLICY";
+static const char usePhysicalDpiEnvVar[] = "QT_USE_PHYSICAL_DPI";
+
+// Reads and interprets the given environment variable as a bool,
+// returns the default value if not set.
+static bool qEnvironmentVariableAsBool(const char *name, bool defaultValue)
+{
+    bool ok = false;
+    int value = qEnvironmentVariableIntValue(name, &ok);
+    return ok ? value > 0 : defaultValue;
+}
 
 static inline qreal initialGlobalScaleFactor()
 {
@@ -249,6 +264,191 @@ static inline bool usePixelDensity()
             qgetenv(legacyDevicePixelEnvVar).compare("auto", Qt::CaseInsensitive) == 0);
 }
 
+qreal QHighDpiScaling::rawScaleFactor(const QPlatformScreen *screen)
+{
+    // Determine if physical DPI should be used
+    static const bool usePhysicalDpi = qEnvironmentVariableAsBool(usePhysicalDpiEnvVar, false);
+
+    // Calculate scale factor beased on platform screen DPI values
+    qreal factor;
+    QDpi platformBaseDpi = screen->logicalBaseDpi();
+    if (usePhysicalDpi) {
+        qreal platformPhysicalDpi = screen->screen()->physicalDotsPerInch();
+        factor = qreal(platformPhysicalDpi) / qreal(platformBaseDpi.first);
+    } else {
+        QDpi platformLogicalDpi = screen->logicalDpi();
+        factor = qreal(platformLogicalDpi.first) / qreal(platformBaseDpi.first);
+    }
+
+    return factor;
+}
+
+template <class EnumType>
+struct EnumLookup
+{
+    const char *name;
+    EnumType value;
+};
+
+template <class EnumType>
+static bool operator==(const EnumLookup<EnumType> &e1, const EnumLookup<EnumType> &e2)
+{
+    return qstricmp(e1.name, e2.name) == 0;
+}
+
+template <class EnumType>
+static QByteArray joinEnumValues(const EnumLookup<EnumType> *i1, const EnumLookup<EnumType> *i2)
+{
+    QByteArray result;
+    for (; i1 < i2; ++i1) {
+        if (!result.isEmpty())
+            result += QByteArrayLiteral(", ");
+        result += i1->name;
+    }
+    return result;
+}
+
+using ScaleFactorRoundingPolicyLookup = EnumLookup<QHighDpiScaling::HighDpiScaleFactorRoundingPolicy>;
+
+static const ScaleFactorRoundingPolicyLookup scaleFactorRoundingPolicyLookup[] =
+{
+    {"Round", QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::Round},
+    {"Ceil", QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::Ceil},
+    {"Floor", QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::Floor},
+    {"RoundPreferFloor", QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor},
+    {"PassThrough", QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::PassThrough}
+};
+
+static QHighDpiScaling::HighDpiScaleFactorRoundingPolicy
+    lookupScaleFactorRoundingPolicy(const QByteArray &v)
+{
+    auto end = std::end(scaleFactorRoundingPolicyLookup);
+    auto it = std::find(std::begin(scaleFactorRoundingPolicyLookup), end,
+                        ScaleFactorRoundingPolicyLookup{v.constData(), QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::Unset});
+    return it != end ? it->value : QHighDpiScaling::HighDpiScaleFactorRoundingPolicy::Unset;
+}
+
+using DpiAdjustmentPolicyLookup = EnumLookup<QHighDpiScaling::DpiAdjustmentPolicy>;
+
+static const DpiAdjustmentPolicyLookup dpiAdjustmentPolicyLookup[] =
+{
+    {"AdjustDpi", QHighDpiScaling::DpiAdjustmentPolicy::Enabled},
+    {"DontAdjustDpi", QHighDpiScaling::DpiAdjustmentPolicy::Disabled},
+    {"AdjustUpOnly", QHighDpiScaling::DpiAdjustmentPolicy::UpOnly}
+};
+
+static QHighDpiScaling::DpiAdjustmentPolicy
+    lookupDpiAdjustmentPolicy(const QByteArray &v)
+{
+    auto end = std::end(dpiAdjustmentPolicyLookup);
+    auto it = std::find(std::begin(dpiAdjustmentPolicyLookup), end,
+                        DpiAdjustmentPolicyLookup{v.constData(), QHighDpiScaling::DpiAdjustmentPolicy::Unset});
+    return it != end ? it->value : QHighDpiScaling::DpiAdjustmentPolicy::Unset;
+}
+
+qreal QHighDpiScaling::roundScaleFactor(qreal rawFactor)
+{
+    // Apply scale factor rounding policy. Using mathematically correct rounding
+    // may not give the most desirable visual results, especially for
+    // critical fractions like .5. In general, rounding down results in visual
+    // sizes that are smaller than the ideal size, and opposite for rounding up.
+    // Rounding down is then preferable since "small UI" is a more acceptable
+    // high-DPI experience than "large UI".
+    static auto scaleFactorRoundingPolicy = HighDpiScaleFactorRoundingPolicy::Unset;
+
+    // Determine rounding policy
+    if (scaleFactorRoundingPolicy == HighDpiScaleFactorRoundingPolicy::Unset) {
+        // Check environment
+        if (qEnvironmentVariableIsSet(scaleFactorRoundingPolicyEnvVar)) {
+            QByteArray policyText = qgetenv(scaleFactorRoundingPolicyEnvVar);
+            auto policyEnumValue = lookupScaleFactorRoundingPolicy(policyText);
+            if (policyEnumValue != HighDpiScaleFactorRoundingPolicy::Unset) {
+                scaleFactorRoundingPolicy = policyEnumValue;
+            } else {
+                auto values = joinEnumValues(std::begin(scaleFactorRoundingPolicyLookup),
+                                             std::end(scaleFactorRoundingPolicyLookup));
+                qWarning("Unknown scale factor rounding policy: %s. Supported values are: %s.",
+                         policyText.constData(), values.constData());
+            }
+        } else {
+            // Set default policy if no environment variable is set.
+            scaleFactorRoundingPolicy = HighDpiScaleFactorRoundingPolicy::RoundPreferFloor;
+        }
+    }
+
+    // Apply rounding policy.
+    qreal roundedFactor = rawFactor;
+    switch (scaleFactorRoundingPolicy) {
+    case HighDpiScaleFactorRoundingPolicy::Round:
+        roundedFactor = qRound(rawFactor);
+        break;
+    case HighDpiScaleFactorRoundingPolicy::Ceil:
+        roundedFactor = qCeil(rawFactor);
+        break;
+    case HighDpiScaleFactorRoundingPolicy::Floor:
+        roundedFactor = qFloor(rawFactor);
+        break;
+    case HighDpiScaleFactorRoundingPolicy::RoundPreferFloor:
+        // Round up for .75 and higher. This favors "small UI" over "large UI".
+        roundedFactor = rawFactor - qFloor(rawFactor) < 0.75
+            ? qFloor(rawFactor) : qCeil(rawFactor);
+        break;
+    case HighDpiScaleFactorRoundingPolicy::PassThrough:
+    case HighDpiScaleFactorRoundingPolicy::Unset:
+        break;
+    }
+
+    // Don't round down to to zero; clamp the minimum (rounded) factor to 1.
+    // This is not a common case but can happen if a display reports a very
+    // low DPI.
+    if (scaleFactorRoundingPolicy != HighDpiScaleFactorRoundingPolicy::PassThrough)
+        roundedFactor = qMax(roundedFactor, qreal(1));
+
+    return roundedFactor;
+}
+
+QDpi QHighDpiScaling::effectiveLogicalDpi(const QPlatformScreen *screen, qreal rawFactor, qreal roundedFactor)
+{
+    // Apply DPI adjustment policy, if needed. If enabled this will change the
+    // reported logical DPI to account for the difference between the rounded
+    // scale factor and the actual scale factor. The effect is that text size
+    // will be correct for the screen dpi, but may be (slightly) out of sync
+    // with the rest of the UI. The amount of out-of-synch-ness depends on how
+    // well user code handles a non-standard DPI values, but since the
+    // adjustment is small (typically +/- 48 max) this might be OK.
+    static auto dpiAdjustmentPolicy = DpiAdjustmentPolicy::Unset;
+
+    // Determine adjustment policy.
+    if (dpiAdjustmentPolicy == DpiAdjustmentPolicy::Unset) {
+        if (qEnvironmentVariableIsSet(dpiAdjustmentPolicyEnvVar)) {
+            QByteArray policyText = qgetenv(dpiAdjustmentPolicyEnvVar);
+            auto policyEnumValue = lookupDpiAdjustmentPolicy(policyText);
+            if (policyEnumValue != DpiAdjustmentPolicy::Unset) {
+                dpiAdjustmentPolicy = policyEnumValue;
+            } else {
+                auto values = joinEnumValues(std::begin(dpiAdjustmentPolicyLookup),
+                                             std::end(dpiAdjustmentPolicyLookup));
+                qWarning("Unknown DPI adjustment policy: %s. Supported values are: %s.",
+                         policyText.constData(), values.constData());
+            }
+        }
+        if (dpiAdjustmentPolicy == DpiAdjustmentPolicy::Unset)
+            dpiAdjustmentPolicy = DpiAdjustmentPolicy::UpOnly;
+    }
+
+    // Apply adjustment policy.
+    const QDpi baseDpi = screen->logicalBaseDpi();
+    const qreal dpiAdjustmentFactor = rawFactor / roundedFactor;
+
+    // Return the base DPI for cases where there is no adjustment
+    if (dpiAdjustmentPolicy == DpiAdjustmentPolicy::Disabled)
+        return baseDpi;
+    if (dpiAdjustmentPolicy == DpiAdjustmentPolicy::UpOnly && dpiAdjustmentFactor < 1)
+        return baseDpi;
+
+    return QDpi(baseDpi.first * dpiAdjustmentFactor, baseDpi.second * dpiAdjustmentFactor);
+}
+
 void QHighDpiScaling::initHighDpiScaling()
 {
     // Determine if there is a global scale factor set.
@@ -259,8 +459,6 @@ void QHighDpiScaling::initHighDpiScaling()
 
     m_pixelDensityScalingActive = false; //set in updateHighDpiScaling below
 
-    // we update m_active in updateHighDpiScaling, but while we create the
-    // screens, we have to assume that m_usePixelDensity implies scaling
     m_active = m_globalScalingActive || m_usePixelDensity;
 }
 
@@ -312,7 +510,7 @@ void QHighDpiScaling::updateHighDpiScaling()
             ++i;
         }
     }
-    m_active = m_globalScalingActive || m_screenFactorSet || m_pixelDensityScalingActive;
+    m_active = m_globalScalingActive || m_usePixelDensity;
 }
 
 /*
@@ -413,22 +611,8 @@ qreal QHighDpiScaling::screenSubfactor(const QPlatformScreen *screen)
 {
     qreal factor = qreal(1.0);
     if (screen) {
-        if (m_usePixelDensity) {
-            qreal pixelDensity = screen->pixelDensity();
-
-            // Pixel density reported by the screen is sometimes not precise enough,
-            // so recalculate it: divide px (physical pixels) by dp (device-independent pixels)
-            // for both width and height, and then use the average if it is different from
-            // the one initially reported by the screen
-            QRect screenGeometry = screen->geometry();
-            qreal wFactor = qreal(screenGeometry.width()) / qRound(screenGeometry.width() / pixelDensity);
-            qreal hFactor = qreal(screenGeometry.height()) / qRound(screenGeometry.height() / pixelDensity);
-            qreal averageDensity = (wFactor + hFactor) / 2;
-            if (!qFuzzyCompare(pixelDensity, averageDensity))
-                pixelDensity = averageDensity;
-
-            factor *= pixelDensity;
-        }
+        if (m_usePixelDensity)
+            factor *= roundScaleFactor(rawScaleFactor(screen));
         if (m_screenFactorSet) {
             QVariant screenFactor = screen->screen()->property(scaleFactorProperty);
             if (screenFactor.isValid())
@@ -441,13 +625,15 @@ qreal QHighDpiScaling::screenSubfactor(const QPlatformScreen *screen)
 QDpi QHighDpiScaling::logicalDpi(const QScreen *screen)
 {
     // (Note: m_active test is performed at call site.)
-    if (!screen)
+    if (!screen || !screen->handle())
         return QDpi(96, 96);
 
-    qreal platformScreenfactor = screenSubfactor(screen->handle());
-    QDpi platformScreenDpi = screen->handle()->logicalDpi();
-    return QDpi(platformScreenDpi.first / platformScreenfactor,
-                platformScreenDpi.second / platformScreenfactor);
+    if (!m_usePixelDensity)
+        return screen->handle()->logicalDpi();
+
+    const qreal scaleFactor = rawScaleFactor(screen->handle());
+    const qreal roundedScaleFactor = roundScaleFactor(scaleFactor);
+    return effectiveLogicalDpi(screen->handle(), scaleFactor, roundedScaleFactor);
 }
 
 QHighDpiScaling::ScaleAndOrigin QHighDpiScaling::scaleAndOrigin(const QPlatformScreen *platformScreen, QPoint *nativePosition)